The present invention relates to amplifiers, and in particular, to techniques for reducing distortion in switching amplifier circuits and methods.
A switching amplifier, sometimes referred to as a class D amplifier, is an amplifier where the output transistors are operated as switches. One example of a transistor used in switching amplifiers is a MOSFET. When the transistor is off, the circuit behaves like an open circuit so the current is zero. When the transistor is on, the voltage across the transistor is ideally zero. In practice, the voltage is very small. Since the equation for power is P=V*I, the power dissipated by the amplifier is very low in both states. This increases the efficiency, thus requiring less power from the power supply and allowing smaller heat sinks for the amplifier. For example, the increased efficiency translates into benefits such as longer battery life. The decrease in the size of the heat sinks lowers the weight, cost and size of the amplifier. Example applications where these advantages would be useful are portable battery-powered equipment such as cellular technology or portable music players.
FIG. 1 illustrates a block diagram of a switching amplifier 100. A continuous input signal is received by a modulator 101 and converted into a train of pulses. The input signal is transformed into a stream of pulses where the pulse characteristics are linked to the amplitude of the input signal. For example, within each period, the duty cycle of a pulse may be proportional to the amplitude of the input signal. For instance, if the input signal received is constant at zero, the duty cycle of the output pulses may be 50%. If the input signal received is highly positive, the duty cycle of the output pulses may be near 100%. Conversely, if the input signal received is highly negative, the duty cycle may be near 0%.
The modulated signal is then amplified in a switching output stage 102. Since the modulated signal is represented by a train of pulses, the output transistors operate like  switches. This enables the transistors to have zero current when they are not switching and a low voltage drop across the transistors when they are switching.
The amplified signal generated by output stage 102 then enters a low pass filter 103 before entering a speaker 104. The low pass filter translates the modified amplified signal back into a continuous signal. A typical filter is an LC filter, for example. The resulting amplified continuous signal may be provided to a speaker and translated into sound. The benefits of low pass filters include minimizing electromagnetic interference (“EMI”) in the amplified signal.
FIG. 2A illustrates the output stage of a typical class D circuit 200. A pulse width modulated signal is amplified by amplifier 201. Similarly, another pulse width modulated signal is amplified by amplifier 202. The amplifiers are coupled to speaker 203. As mentioned above, a filter may be included between the amplifiers and speaker to filter the waveform. The signals transmitted to the speaker are typically voltages and currents. The circuit illustrated in FIG. 2A is an open loop system and may experience distortion. Varying speaker impedance, power supply loading, and offsets due to power amplifier component mismatches are some of the factors which may influence distortion in an open loop system. The audio speaker is an electromechanical device which has input impedance which varies as the cone of the speaker is deflected through its range of motion, and if the amplifier does not compensate for the changing impedance, the resulting signal may contain distortion. Also, during bursts of sound, the amplifier power supply may droop causing the output signal to distort. Additionally, there may also be offsets between the positive and negative portions of the audio signal due to inherent mismatch of components within the power amplifier circuits, and this may cause distortion as well.
Another disadvantage of switching amplifiers is the crossover distortion that may appear in signals transmitted from the amplification stage. For example, crossover distortion in a differential system with the center point at zero volts will occur when the differential signal transitions from positive to negative voltages. However, the crossover point may occur at any voltage depending on system design. FIG. 2B illustrates waveforms within a switching amplifier. The input sinusoidal waveform is shown on plot 210. The crossover point in this example is the zero crossing of the sine wave is at 211. The peak of the sine wave, +v, is at 212 and the trough of the sine wave, −v, is at 213. Thus, the zero crossing is the transition point where the input signal transitions from a negative region of 0 to −v to a  positive region of 0 to +v, or vice versa. Small signal pulses generated from modulator 101 of FIG. 1 surrounding the zero crossing may not be amplified in the output stage 102 of FIG. 1 due to the inherent transmission delay in the power amplifiers. Plots 220 and 230 illustrate the two amplified signals generated from the output stage for the two channels. Zone 250 illustrates the area surrounding zero crossing wherein small modulated pulses may not be translated by the output stage. For example, if the signal pulse received by the output stage was 5 ns and the transmission delay of the output stage is 10 ns, the 5 ns pulse will not be transmitted to the output terminal of the output stage. This will result in distortion. Plot 240 illustrates the signal generated by low pass filter 103 in FIG. 1. Due to the missing pulse widths in output signals 230 and 240, a point of discontinuity in the output waveform exists. This output distortion is illustrated by the flat area 241. This discontinuity, also known as crossover distortion, may increase the total harmonic distortion and noise of the output signal.
Thus, there is a need for switching amplifier feedback in low distortion systems. The present invention solves these and other problems by providing switching amplifier feedback circuits and methods.